The invention relates more particularly to a ventilation device comprising at least one organ for centrifugal propulsion of air intended to route the air through a distribution circuit towards the passenger space of the vehicle. The centrifugal propulsion organ includes an annular turbine provided with blades which is driven in rotation about an axis of rotation by a motor and which is generally arranged at the centre of a volute-shaped casing. The air is taken by the turbine axially from outside the casing and is driven along a passage formed by the casing to an evacuation orifice, or radial air delivery mouth communicating with the distribution circuit.
The passage is delimited by the outer surface of revolution of the turbine, defined by the outer section of the blades, and the inner face of the peripheral wall of the casing shrouding the turbine blades at a distance. The radial profile of the peripheral wall of the casing evolves in a volute so as to cause progressive variation of the separation distance between the section of the blades and the inner face of the peripheral wall. The peripheral wall therefore forms a spiral around the turbine in the direction, the said spiral evolving in the direction of rotation of the turbine.
To define the shape of the volute, an angle of expansion of the volute is defined which corresponds, at a determined point of the volute, to the angle formed between the tangent to the volute and the tangent to a circle passing through this point, the centre of the volute and of the said circle here being formed by the axis of rotation of the turbine.
Generally, travelling through the volute in the direction of rotation of the turbine, it is found that the angle of expansion is substantially uniform, which leads to a regular increase in the distance between the outer section of the blades of the turbine and the inner face of the peripheral wall.
For the design of such a ventilation device, it is necessary to deal with different parameters such as the efficiency of the device, the yield and flow-rate of air provided by the turbine, the overall size and the relative dimensions between the turbine and the casing, as well as the noise pollution generated. It is generally desirable to have the smallest possible size of the device to optimise the available space, and it is desirable to avoid as much as possible the creation of zones of turbulences generating noise pollution.
Various solutions and different compromises have been proposed in the devices of the prior art, but they do not allow an entirely satisfactory result to be obtained in terms of size and of sound level.